A preliminary study on the length of incubation needed to maximize guanidination of lysine in distillers dried grains with solubles (DDGS) and in pig ileal digesta

In heat-processed feed ingredients, the concentration of bioavailable lysine (Lys) may be estimated by measuring the amount of ileal digestible Lys that has a free ɛ–NH2 (i.e., not bound to reducing sugars). This Lys, called ileal digestible reactive Lys, is determined by measuring the reactive Lys in the feed ingredient and in ileal digesta of pigs fed the feed ingredient. A procedure used to measure reactive Lys in feed ingredients and in ileal digesta samples is the homoarginine procedure, which consists of a guanidination to convert the Lys that has a free ɛ–NH2 group into homoarginine. However, the incubation time needed to maximize the guanidination can vary among types of protein and there is no information on the time needed to maximize guanidination in ileal digesta from pigs. Thus, an experiment was conducted to determine the optimum time of incubation needed to guanidinate Lys in maize distillers dried grains with solubles (DDGS) and in ileal digesta from a pig fed a diet containing maize DDGS as the sole source of Lys and other amino acids. A DDGS sample was guanidinated in 0.6 M O-methylisourea reagent at pH 11.4 for 1, 3, 6, or 9 days. Ileal digesta from a pig fed a diet containing DDGS was also incubated in 0.6 M O-methylisourea for 1, 3 or 6 days. The incubation time that resulted in maximal amount of Lys that had been converted to homoarginine was then determined. Results showed that in DDGS, the optimum incubation time in DDGS was 3.2 days (P<0.05). For ileal digesta, breakpoint analysis showed that the optimum incubation time was 3.7 days (P<0.05). In conclusion, the optimum conversion of Lys to homoarginine in DDGS and in ileal digesta requires an incubation time of 3.2 and 3.7 days, respectively, if 0.6 M O-methylisourea is used for incubation at a pH of 11.4.